Specialized viewing systems can greatly enhance the observation of objects in turbid media. For example, such systems are useful in seawater for salvage and recovery operations or for target detection. Such a system could also be used to allow aircraft or ships to navigate in fog or other adverse weather conditions.
Turbid media viewing systems are designed to avoid the deleterious effects of scattering, that is, to separate information containing light from non-information containing light. Conventionally, a method of accomplishing this is to separate the light source from the receiver. The greater the separation, the less scattered light is detected by the receiver. Usually, the source and receiver are as far apart as is permitted by the size of vehicle upon which they are mounted.
Another method of avoiding the effects of scatter is range gating. In range gating, a pulsed light source is used and the receiver is activated only upon receiving the light pulse that is reflected from the target.
In another method, synchronous scanning, a very narrow source beam illuminates a small portion of the target and scans the target. The receiver has a very narrow field of view and is synchronized to follow the scanning source. This technique minimizes the volume of illuminated water that is in the field of view of the sensor at a given time and thereby enhances the signal-to-noise ratio which improves performance of the system.
There are a number of disadvantages to these prior art methods or combinations thereof. Conventional range-gated techniques illuminate the entire target with a single pulse. The light is spread over the entire target and an extremely intense, high power pulse is therefore required to illuminate any target that is a substantial distance from the system.
In conventional scanning systems, extremely sophisticated and complex mechanical synchronization techniques are required to keep the small field of view of the receiver coincident with the small illuminating spot as it scans the target.
Therefore, it is desirable to have a system for use in turbid media that reduces the need for very high power light pulses and eliminates the mechanical synchronization problems between source and receiver while retaining the benefits of both range gating and spatial gating.